Dr. J. B. Haycraft. 



histologist of ten or twenty years ago felt bound to assume that the 

 fibril was a very complicated structure, and he never doubted that a 

 muscular fibril consists of a series of alternating and recurring struc- 

 tures. It then became his duty to find out what these structures really 

 might be, and what part they play during a muscular contraction. 



The lines of Dobie are often seen as narrow dark bands, which 

 were believed to be membranes (Querwand), and it was held that 

 these membranes separated up the fibrils into little boxes (Mnskel- 

 kastchen), so that a fibril consists, according to these authorities, of a 

 series of little boxes, joined end to end, containing the substances 

 whose position was marked by the other stripes. Certain of these 

 stripes (the dim ones) were considered to mark the position of solid 

 or relatively more solid substances, and the other stripes (the clear 

 ones) to consist of fluid or relatively less solid substances. The 

 appearance of the stripes, the staining, and their action on polarised 

 light gave, at any rate, some colour to this hypothesis, for the dim 

 stripes appear to have more substance than the clear stripes ; they 

 appear to stain with reagents, and to doubly refract light, which 

 latter property is certainly seen in some solids. The light stripes, 

 on the other hand, appear deficient in substance and solidity, they 

 stain less readily, and they simply refract light (a property common 

 to all liquids, and some solids). 



The Muskelkastchen hypothesis seemed, therefore, feasible enough, 

 and having under their microscopes little boxes containing more 

 solid and less solid parts in alternating layers, Krause, Merkel, 

 Engelmann, and others, sought to explain, each in his own way, 

 the most obvious phenomenon of contractility, namely, the shorten- 

 ing and thickening of the contractile tissue, as beiug due to the 

 interaction of these structures. 



Histologists are accustomed to observe osmotic changes, the swelling 

 up and the shrinking of red blood-corpuscles, for instance, and to see 

 the resulting alterations of form. Under these circumstances it was 

 not unnatural for them to suppose that during contraction the more 

 solid parts of the Mnskelkastcben imbibed fluid from the less solid 

 parts, in such a way as to alter the shape of the muscle box, making 

 it shorter and thicker, and causing, in consequence, the whole fibre 

 to change in the same way. 



In apparent support of this theory, the stripes in the muscle 

 boxes change their relative thickness, and alter in appearance, in the 

 manner so carefully described by these observers. There seems, in 

 fact, only one objection to this osmotic theory which would at once 

 present itself to the eye of the critical observer ; it is the time take 

 by the process, for osmotic changes are slow in their very nature, ant 

 the muscles of an insect's wing can contract over a hundred times a 

 second. 



